Halogen containing polymers, especially those containing chlorine, are used widely and have great commercial significance. Polyvinyl chloride (PVC), in particular, is used in packaging, siding, pipe, and many extruded shapes. Such large scale and diverse use of the halogen-containing polymers depends upon the incorporation therein of good heat and light stabilizers. PVC, for example, is known to have a tendency to degrade upon prolonged exposure to heat and light during processing and use. Darkening or other color change and the loss of tensile, flexural, and impact strengths are the results of such degradation. Unless good low-cost stabilizers are available for addition to the polymer composition, the service life of articles made from the composition will be limited, and its use severely restricted, as will be the conditions for making it.
One particularly troublesome form of degradation occurs when the polymer composition is processed into articles by methods employing heat to melt or soften the polymer. A color change can occur during the first few minutes at high temperatures (e.g., from about 175.degree. to about 200.degree. C.) and it is commonly referred to as early color or early discoloration. The avoidance of such early color is notably important in the manufacture of plastic pipe and siding. It is, of course, also important to prevent or reduce discoloration and deterioration of the polymer during extended exposure to high temperatures, which can lead to sudden and catastrophic degradation into a pitch-like abrasive material and cause the formation of corrosive materials such as HCl inside the fabricating equipment. The inner, highly polished surfaces of the equipment can thus be rendered essentially useless.
Attempts have been made to replace organo-tin stabilizers at least in part with other organo-metal compounds such as zinc mercaptoesters. The complete replacement of organo-tin compounds by zinc mercaptoesters such as [C.sub.8 H.sub.17 OC(.dbd.O)CH.sub.2 S].sub.2 Zn in combination with alkali metal salts of organic acids is taught in Japanese Kokai 75 89451. In U.S. Pat. No. 4,515,916, Molt teaches that a composition comprising a zinc mercaptoester, a basic inorganic alkali metal or alkaline earth metal compound, and, optionally, a substituted dihydropyridine is useful as a stabilizer for halogen-containing polymers.